Since todays (12.10.2015) release of the development branch (1.53.132856) you are able to stand up and shoot in the driving/facing direction of the offroad.
That is a nice feature, but it is too limited (also take a look at #0021011)
You can only turn your torso 180 degrees. It seems like you have glue on your shoes and a stiff back.
If I am able to stand on the back of an Offroad, I am able to turn around 360 degrees.